This invention relates generally to methods for ion vapor deposition of thin metallic films, and more particularly to a novel method for depositing films of selected metals or semiconductor materials by dissociatively photoionizing the corresponding halide compound in an electric field.
Techniques in general use for depositing metallic or semiconductor films from the vapor phase comprise production of ions for deposition from the vapor of the specific element to be deposited. Consequently, the extremely high temperature required to vaporize a metallic or semiconductor material for deposition produce contaminant ions by chemical reaction with the equipment housing or by outgassing of the reactor wall, and a mass selection step in the deposition process may be required to remove the contaminant ions.
The present invention provides a novel method for vapor depositing metal or semiconductor films, particularly of group III, IV and V metals and semiconductor materials. It has been previously demonstrated (A. Terenin et al, Phys Z Sowjetunion, 2, 299 (1932)) that atomic metal-halogen ion pairs may be produced by irradiating the vapor of polar metal halide diatomic molecules in a narrow region of the ultraviolet near 200 nanometers (nm). According to the present invention, a metal halide compound is vaporized in a photoionization cell and subjected to a uniform electric field of predetermined field strength. The halide vapor is irradiated by light of preselected wavelength to dissociatively photoionize the vaporous halide molecules; the (positive) metallic ions so produced are deposited as a film on a substrate comprising the cathode of the field-producing electrostatic lens. The deposition method of the invention utilizes the large ultraviolet photoionization cross sections characteristic of certain halide molecules to produce large and easily controlled densities of positive ions for plating. The invention is a substantial improvement over existing deposition methods: First, the use of the metal halide as the donor molecule ensures that the dissociative photoionization process produces only one positive ion (the metallic ion) which minimizes the incorporation of foreign species into the resulting film and eliminates any need for a mass selection step in the process. Second, since the metal halide compound generally has a much higher photoionization cross section than that of the metal, and since the halide compound is considerably more volatile than the metal, large numbers of the metallic ions may be produced at a moderate irradiation intensity, the deposition can be performed at much lower temperatures than with conventional methods, and the substrate can be conveniently separated from the ion production region in the deposition chamber. Third, since the process of the invention produces charged ions of the metal to be deposited, selective area disposition is readily attainable by electrostatic control, and, consequently, writing on a substrate is achievable which may be useful in producing crystalline or compound semiconductor films or in growing and doping materials, such as indium phosphide, which decompose at relatively low temperatures.
It is, therefore, a principal object of the invention to provide an improved method for the deposition of metal or semiconductor films.
It is a further object to provide a low temperature process for the deposition of thin films of selected metals and semiconductor materials.
These and other objects of the present invention will become apparent as the detailed description of certain representative embodiments thereof proceeds.